Particulate matter is currently a topical subject. Particulate matter is understood as very small particles whose sizes (aerodynamic diameters) amount to less than 10 μm.
Particulate matter is characterized by PM10, PM2.5 and PM1. The dust particle size characterized as particulate matter PM10 contains 50% of the particles having a diameter of 10 μm, a higher proportion of smaller particles, and a smaller proportion of larger particles. The dust particle size characterized as particulate matter PM2.5 contains 50% of the particles having a diameter of 2.5 μm, a higher proportion of smaller particles, and a smaller proportion of larger particles. PM2.5 is a subset of PM10. The diameter for PM1 is 1 μm.
Since particulate matter carries health risks, it is important to avoid or reduce particulate matter. To be able to take and monitor the correct measures, it is absolutely necessary to know the size distribution of the particular matter and the mass percentages of the particular matter types PM10, PM2.5 und PM1.
The demands on the determination of particulate matter are increasing all the time since the filter systems of emitting plant such as coal-fired power stations are becoming better and better so that the concentrations are falling; however, at the same time, the limit values are becoming ever stricter, e.g. due to the revision of the directive on national emission ceilings (NEC directive) approved in 2016 that added a commitment for 2030 according to which the German PM2.5 emissions have to fall by 43% over 2005 by 2030. Smaller and smaller concentrations therefore have to be determined and analyzed.
Continuously measuring automatic measurement systems (AMS) are used worldwide to monitor particulate emissions or particulate immissions. In practice, an AMS has to be calibrated by means of a gravimetric standard reference method (SRM) at its installation site to convert the information of the AMS only on scattered light into real dust mass concentrations. This calibration becomes more and more difficult with increasing inaccuracy for small dust mass concentrations (˜1 mg/m3). Enormous efforts are being made worldwide, e.g. by an artificial increase of the dust concentration by injection of additional dust or by a reduction of the emission control effect, to thus move the values of the SRM measurement above the detection threshold. Alternatively, legislation in Europe permits an extension of the measurement time to better quantify very small dust concentrations or it allows the use of so-called substitutes in exceptional cases whose particle size distribution is very similar to that of the exhaust gas.
It is disadvantageous that these methods do not represent any real solutions for the measurement problem since they on the contrary increase the costs for the system operator for the initial calibration or for the regular functional test. In addition, it is not possible to obtain any size-resolved information on the dust concentration with current AMS.
An analysis device is known from EP 0 391 256 B1 for determining a particle size distribution in which the 90° scattered light technology is used. The particle size is determined using laterally scattered light. The scattered light is detected and is measured as a voltage signal. A conclusion can be drawn on a particle size from the voltage signal by means of a calibration function. The number of particles is obtained via the same signal in that pulses of the signal are counted. The particle mass concentration separated by particle size is calculated by volume integration of the measured particle number size distribution curve.
The greatest disadvantage of this 90° scattered light technology is that the scattered light of single particles is measured. Single particles are required for this purpose that have to be present in the measurement volume and thus have to be introduced into the measurement volume as single particles. This is laborious and additionally brings about further problems such as low partial pressures and a supply of carrier gas (for dilution) and thus a possible falsification of the measurement results.
A further technology for particle size analysis is laser light diffraction (laser diffraction technology). The interaction of laser light with particles produces characteristic scatter patterns. These scatter patterns depend on the particle size, on the optical properties of the particles, on the dispersion, and on the wavelength of the incident light. Large particles scatter light more in the direction of small scattering angles. An analysis device therefore requires a high resolution in the forward direction, but also in the direction of large scattering angles for light scattered laterally and back. A known analyzer is the HORIBA LA-960 that has a multi-element ring detector in the forward direction and a plurality of further single detectors toward the side and in the rearward direction and thus detects large areas of the total measurement region. In addition two light sources having different wavelengths (650 nm and 405 nm) are used, whereby the sensitivity for smaller particles (nanoparticles) is increased.
The greatest disadvantage of laser diffraction technology is that the device does not output any absolute particle size distribution (in physical units, e.g. μg/m3). Such devices instead deliver a relative size distribution function and a cumulative size distribution from 0 to 100%. Neither a particle mass concentration nor mass percentages of the particular matter, i.e. PM1, PM2.5 or PM10, can thus be obtained.